Piezoelectric apparatus



July 9, 1940- c; K. GRAVLEY y 2,207,539

PIEZ'OELECTRIC. APPARATUS I Filed July 2e, 1939 2 Sheets-Sheet 1 V ..5 y @n .M mA N n o N6., r EUA T V www Pfc.` my

July 9, 1940.

c. K. GRAVLEY PIEZOELECTRIC APPARATUS Filed July 26, 1959 z sheets-sheet a l Arm/mask Passed .my a, 1940 .UNITED sTA'ras PATENT OFFICE icmhzifsgl-oseml No. 286,5"19v A1o claims (c1. 1v1-szw) l This invention relates to piezoelectric transducers in general and moreparticularly to a crystal mounting and 'lever arrangementv suitable foruse in piezoelectric devices of .various 5 types and especially in a pickup device for measuring fluctuations in fluid pressure.

An object of the invention is .to provide a lever arrangement suitable for use in piezoelectric transducers' of various types. l Another object is to provide a lever system having low stiffness for motion in a desired direction and relatively high stiifness for motion in other directions.

Another object is to provide a pressure pickup which is relatively insensitive to all vibrations except those which occur inthe fluid mediumactuating the device.

' A nother object is to provide-a device for meas-5 'uring fluctuations in .fluid pressure which embodies means permitting the device to be altered readily to accommodate a wide range of static pressures.

For a fuller understanding of the nature and objects of the invention, reference should be hadl to the following detailed description taken in connection with the accompanying drawings in which,

Fig. 1 is a plan view of a pressure pickup constructed in accordance with the invention,the`

view being taken on line I,'I of Fig. 3.

Fig. 2 is a sectional plan view ofthe pickup taken along line 2, 2 of Fis. 3.

Fig. 3 is 'a sectionalview along line 2, 3 of F18. 2 Fig. 4 ls a sectional view of a modii'ied form of the pressure pickup. Fig. 5 is a sectional view of la. loudspeakerl embodying the invention.

Figs. 6 vand 7 show a simplified form of the in 0- vention suitable 'for use 'with a bender type piezoelectric unit in which Fig. 6 is a plan view,

and Fig. 'l is a sectional view taken on line 1, 1

'of Fig. 6.

Figs. 8 and 9 'show a device embodying another modified construction which is particularly adapted for use as a phonograph pickup or record cutter, to be used in hill-and-dale type recordings, Fig. 8 being a vertical sectional view taken along line 8, 8 of Fig. `9, and Fig. 9 being a bottom plan view of the device.

lA pickup device for measuring fluctuations -in fluid pressure (for example, a device for measuring fluctuations in pressure in an oil or gas pipe line) should be .capable of indicating relatively 5l slow pressure changes. A piezoelectric crystal pickup is particularly well suited for this Purpose because the voltage generated by a piezoelectric unit is approximately proportional to the displacement. However, in orderto prevent false indications it is essential that the piezoelectric 5 unit be mounted so that physical shock or vibration of the case of the device, or of its support will not cause any appreciable voltage to be generated.' This can be accomplished by the type of construction illustrated in Figs. 1 to 4 inclusive. 10 In Figs. 1 to 3 a square piezoelectric blmorph unit I of the torque type such as is' disclosed in United States patents numbered 2,105,011 and Reissue 20,680 is connected at its four corners to wire drive links 2, 2. 'I'his may be done conveniently 15 .byfirst soldering the drive links 2, 2 to corner clips 3, 3 and then cementing the corner clips to the-piezoelectric unit. 'I'he other ends of the drive links are soldered to levers 4, 4`and B, l. Each pair of levers is embodied in a single piece 20 of metal tubing iiattened at its midpoint as shown in the drawings. Levers made in this way are very desirable because they are rigid, yet have low inertia. The length of tubing is flattened at its midpoint to obtain flexibility and g5 l to facilitate attachment of a suitable drive rod. Levers 5, 5 are levers oi the first class, while levers 4, 4 are levers of the second class. The ratios of ali of these levers should preferably be the same. 'Ihe levers are fulcrumed on wires 30 B, B which serve not only as supports for the lever mechanism but indirectly serve as supports for the piezoelectric unit. It is desirable to solder thefulcrums to both the levers and the assembly plate. The two lengths of` tubing constitut- 35 ing the levers are connected together at their flattened midpoints by means of an adjustable drive screw 8 threaded into a lock nut I0 and intoY nut '9 which is soldered to the levers 5, B. 'Ihe entire mechanism is mounted in a rigid case 40 adapted to enclose opposite -faces of the assembly plate 1 and consisting of a cup shaped base cap II, top cap I2, nut I3, and plug I4.

. The base cap 'is secured to assembly plate 1 by means of countersunk screws I5, vI5 a gasket 45 I6 being disposed between their adjacent surfaces to secure a substantially air-tight joint. The drive links 2, 2 are surrounded by flexible rubber 'bushings I1, I1 where they pass through assembly plate 1. Likewise all other openings in 50 plate 1 are closed and made substantially airtight so that cavity I8 in which the piezoelectric unit I is disposed is effectively sealed against the ingress o'f moisture. Top cap I2 is secured to assembly plate 1 by means of screws I8 which 55 A' io illled with compressed packing material 2li. The

pass through plate l and are threaded into the wall of base cap il. There are four recessed portions 2t in the top cap which extend radially outwardly' from the central circular opening 2i, and which are adapted to rit around the ends ci the levers l, l and 5, ii. 'Ihisconstruction permits thicker walls on the cap than would result if the central opening 2i were made large enough to fit around the levers. The greater thicmess produces increased rigidity of top cap i2. The

central opening 2i is closed at one lend by nut i3 which is threaded thereinto. The nut is provided with an annular external shoulder 22 which engages a surface of cap i2 and limits the depth to which nut i3 extends into the central cavity of cap i2. The nut iii is also provided with a threaded opening 23 which terminates in a hat faced ange 26 on which is disposed a sheet metal diaphragm 2&3. Plug it is adapted to be threaded into opening i223 and press diaphragm 25 iirmly against nange tl. 'The plug is recessed adjacent the outer periphery 'of its inner end to form an annular cavity Ztl which is substantially plug is provided with a central opening 28 which is closed at one end by diaphragm 25 and which is threaded at its other end to receive tube 2@ or other conduit which communicates with a source of uid pressure.

Drive screw S is adjusted with respect to levers d, f3 and 5, 5 so that its head bears against diaphragm 25 with moderate force when the latter is firmly clamped in place upon ange 2d. When the device is assembled in this way, it will b e understood that iluid pressure admitted through tube 29 deforms the diaphragm toward plate l, displacing drive screw t and actuating levers l, and 5, 5 to flex the piezoelectric unit i. Thereafter, fluctuations in the pressure acting on the diaphragm cause the unit to vibrate in such a manner that an opposite pair of corners will move in one direction, while the other pair, of corners move in the opposite direction. In other words, the unit is twisted. When the tonque is iiexed in this manner it generates a potential diiference at its terminals which is approximately proportional to the displacement. The potential dierences thus generated may be caused to actuate an appropriate electrical instrument connected to the terminals of the unit. Connections to the terminals are made through lugs 3i?, dll and lugs 3|, 3i which are secured on opposite faces of plate l by a rivet, the lugs and rivet being insulated from the plate by anged insulating bushings 32, 32. The terminals of the piezoelectric unit are connected to lugs 3Q, 3G while terminals of a shielded cable 33 may be soldered to lugs 3i, 3i the cable being secured to the device by soldering its woven metal shield to a clamp 3d. It has been found desirable to dispose the drive links v2, 2 as nearly parallel to each other as practicable in order to eliminate as far as possible any end Wise component of force due to sidewise vibration of the piezoelectric unit such as might be induced by mechanical vibration of the pickup assembly as a whole. A small pad 35 of viscoloid or other suitable damping material is placed between the crystal unit and the assembly plate l in order to reduce to a minimum sidewise vibration of the crystal.

If the pickup device is designed to have rela tively large lever-ratios and a ,relatively 'thick dia phragm then the displacement of the diaphragm per -unit of uid prsure will depend substan= tially only on the constants of the diaphr.

accesso This may be advantageous for applications where known pressures are not available for calibration. The displacement oi the diaphragm as a function of fluid pressure may be calculated from its dimensions and knownconstants without con- 5 siderin'g the eiect of the lever-crystal mechanism. The lever-crystal mechanism may be calibrated in terms of voltage output per unit displacement of drive screw and this calibration may be combined with the calculated 'data for the lo diaphragm to obtain a calibration for the comh plete device in terms of voltage output as func tion of fluid presure.

Fig. d shows a modified construction which provides even greater rigidity than is found in the la construction shown in Figs. l to 3. The device shown here comprises a cup-shaped base cap tti disposed adjacent one face of assembly plate it@ with a gasket d@ between the adjacent suriaces of the two members, plate t9 being secured to tu propriate diameter, while opening d5 extends between opening @E3 and the base oi the top cap, and has as small a diameter as practicable. A diaphragm 38 rests on flange il? and is rmly .clamped thereon by plug t9 which is threaded into opening de. The base of the plug has an annular recess 5@ adjacent its circumference in which is disposed packing material 5i. The base l of the plug is also provided with a shallow centrai opening 62 recessed therein and having the same diameter as opening d5. An opening 53 5.@

extends between opening 52 and the opposite face of the plug, and is threaded to receive a tube 5d which communicates with a source of uid pressure.

A lever a-nd piezoelectric unit assembly are disposed within the cavity 55 formed between plate 5 39 and cap te. The assembly comprises two pairs of intersecting levers 5t, 56 and 5l, 5l each pair being made of a single length of tubingI attened at its midpoint, and supported on wire fulcrums 58, 58 soldered to the levers and also soldered to plate 89. One pair of levers is of the iirst class, while the other pair is of a dierent class, second class levers being shown in the drawing. Each lever is provided with a connecting. link 59 which is soldered thereto at one end andwhich is se- 55 cured at its other end to a corner of a piezoelectric torque unit in the same manner as previously described in connection with the con-I struction shown in Figs. 1 to 3. It will be noted that the levers are disposed between the piezoelectric unit and the platet. The levers are fastened together at their midpoints by a threaded drive-pin d@ engaging nuts 6i and 62. The drive pin passes through an opening St in plate 3s having a exible rubber bushing @il se- 5 cured therein and eectively closingl the space between the drive 4pin it@ and the walls of the opening es. The other end oi drive pin @t is threaded into a nut d5 and a lock nut SS. The nut d5 may be adjustably positioned on ,drivefpin t@ so as to lc'sgary nrmly against diaphragmdt, andv may be retained in adjustment by lock nut S6.

Electrica-l connections with the terminals of the piezoelectric unit are made through apair of electric unit are secured to lugs 01 while .the ter- A ,aoasse y minals of.a shielded cable' 1| are soldered to lugs 68. The woven metal shield 12 of the cablel 1| is soldered to clamp 10. The cable passes through an opening 14 recessed in the base of top cap 42.

It should b e noted that both of the construc-l tions shown in Figs. 1 to 4 .permit diaphragm Il or diaphragm 48v to be replaced readily without drive pin and lever assembly. Thus it is possible to replace the diaphragm withone of greater or less thickness and thereby alter the device readily so as to make it respond to fluctuations in a wide range of static pressures.

Both of the above constructions may be designed to employ a piezo-electric unit of the bender type in place of the torque type unit described, it being necessary merely to connect the levers to the midpoints of opposite sides of the bender unit instead of to the corners of the torque unit. This change is made necessary by reasonof the disposition of the crystalline axes of the unit with respect to its boundary edges, as will be explained below in more detail.

It will be noted that the constructions shown in Figs. 1 through 4 have a symmetrical lever arrangement whlch produces a balance of forces in the system. It has been found that this type of structure, when incorporated in a housing having appreciable rigidity, is substantially insensitive to vibrations applied elsewhere than to the drive rod. By virtue of this inherent property. the structure is particularly suited for use in pressure pickups of the type described, especially since such pickups are likely to be mounted in use on a pipeline or mechanical device which is in a state of rather violent vibration.

The invention is not limited kinsita application to devices such as shown in Figs. 1 to 4, since a multiple lever assembly may be used in conjunction with a piezoelectric unit in a variety of ways. For example. Fig. 5 shows a loudspeaker which utilizes the lever and plate assembly of. Figs. 1 to 3. In this loudspeaker construction, the assembly comprising base cap assembly plate 1, piezoelectric unit I, levers 4, 4, and 0, l, and the other associated elements carried on assembly plate 1 of Figs. 1 to 3 are employed as a unit and disposed upon a suitable support such as the vcup shaped member 10 in Fig. 5, being secured thereto by screws18. A drive pin 11 is secured at one end to the levers .4, 4 and l, l at their flattened midpoints. The drive pin is secured at its other end to a sound radiating diaphragm such as a cone 18 supported in the usual manner around its periphery. In this construction, electrical'impulses of sound frequency are applied to the terminals of the piezoelectric unit causing it to flex and to actuate the levers which in turn actuate the cone. electric torque unit I may be replaced with a piezoelectric unit of the bender type by connecting the drive links of the levers to appropriate points on the bender unit. A

The invention may also be applied in asimplified form in a device such as is shown in Figs. 6 and '7. Here a .base cap 00 is secured to an assembly plate 8| by screws 0I, two gaskets 83 and 84 being disposed between their adjacent As indicated above, piezop surfaces. Two wire fuicrums 0l, 00 are secured as by soldering to the plate 0|, and fulcrum a pair of levers 0l, 4t which are embodied in a single length of tubing flattened at its midpoint. Each lever has a drive link 01 secured to it at a point lying between its fulcrum and the midpoint of the length o! tubing. The drive links are secured to opposite ends of an elongated bending-type piezoelectric unit Il. Supports 08, 00 are disposed on opposite sides of the unit adv jacent its midpoint yand serve to hold the center of the Aunit substantially xed relative to plate 0| and the adjacent wall portion of cap 00. The supports may be made of damping material if desired. The drive links 01 pass through openings 00, 0l in the base plate, the space between the drive linkami the wall of each opening being substantially closed with a flexible rubber bushing Il. The terminals 02 and 03 of the piezoelectric unit pass between the gaskets and 04 and may be connected to any suitable'electric circuit. A drive pin 84 may be secured to the attened portion c2 the tubing composing levers v06, 08 and may be coupled to any suitable mechanical system. It will be understood that if the drive pin is actuated, the ends of the piezoelectric Yunit will be flexed with respect to the center and the unit thereby caused to generate a potential dierence at its terminals. Conversely, potential diiferences may be applied to the terminals of the piezoelectric unit causing it to ilex,l and thereby to actuate the levers and drive pin.

Figs. 8 and 9 illustrate another modified construction: one which is particularly adapted for use in a hill-and-dale type record cutter or phonograph pickup. A record cutter has been shown in the drawings 'and is seen to consist of a cutter arm 00 having a cup-shaped cavity detype is disposed within the cavity and supported at one end by two fixed wire supports |04, `|04 secured to its corners and in turn secured to the assembly plate |00. Disposed on the opposite side of the assembly plate are two wire fulcrums |00, |00 each secured at one end to the assembly plate and at its other end to a lever. The levers |01 'and |04 are formed of a length of tubing bent into an L shape and flattened at the corner oi' the L. Lever |01 is shorter than lever |08 and is secured to fulcrum |00 so 'as to function as a lever ot the ilrst class. Lever |00 is secured to fulcrum |08 so as to function as a lever of a din'erent class. such as the thirdclass lever, illustrated. Each lever has a drive link |09 secured to it at an appropriate point, the link being connected atcits other end to one of the unsupported corners of the piezoelectric unit.

The drive links pass through openings ||0, ||0, in plate |00, the space between thev drive pin and the wall of the opening being closed by a exible rubber bushing A cutting stylus ||2 is secured to the flattened portion of the. levers by means of nuts IIS, III. The terminalsv short sides, so that the unsupported corners which lie ,on opposite sides of the twisting axis move in opposite directions about the axis.

' Since the -levers are of the iirst and third class,

these motions of the corners cause the common point of the levers to move toward or away from assembly plate 00. Thus, when a recording disc is moved under the cutting stylus, electrical impulses applied to the terminals of the piezoelectric unit cause the stylus to cut a hill-anddale type groove in the disc. Conversely, up and down vibrations imparted to a pickup stylus by a hill-and-dale type record twist the piezoelectric unit and cause it to generate a potential difierence at its terminals.

Many other forms of the invention will occur to ythose skilled in the art, particularly when the principles of mechanics are applied in combina-A tion with the principles of operation of piezoelectric units. The principles or mechanics are Well understood and need but iittle mention here. In order to point out the scope of the invention, however, the principles oi operation of bimorph piezoelectric units will be discussed briefly, while reference is made to -the three United States patents numbered 2,105,011, Re. 20,213 and Re. 20,680, for a more extended dis cussion. Bimorph (multiple) units are units of the flexing type and are divided into two classes in accordance with the type of iiexing motion which they exhibit or respond to; namely, the bender type, and the torque type. For example, a torque type unit, when made of Rochelle salt may contain a square or rectangular plate which has been cut so as to have its edges substantially parallel to the major crystalline axes of the Rochelle salt. When this plate is secured to a constraining member, whether the latter be another plate of piezoelectric material .properly oriented with respect to the former or merely a plate of non-piezoelectric material, a bimorph unit results. When a potential difference is applied to the electrodes of the unit, it `twists about its major axes so that parts of the unit move out of the original plane of the unexed unit. In square or rectangular units the greatest motion is at the corners, and the least motion occurs along the axes. Stated in another way, if a square piezoelectric torque unit were supported at its center of gravity and were flexed by applying a potential diierence to its terminals, points lying on the crystalline axes, passing through the center of gravity, (that is, points lying on the lines passing through the midpoints of opposite sides of the piezoelectric plate) would remain stationary and dene a stationary plane. All points of the crystal except those lying on the axes would move out of the plane and the motion would be suchl that a pair of opposite corners would move out of the plane in one direction while the other pair of opposite corners would move in the opposite direction.

The bender type Rochelle salt bimorph unit contains a piezoelectric plate which is cut, if square, so that the crystalline axes are parallel to the diagonals of the square. This plate issecured to another restraining member, as in the torque bimorph, which causes the assembly to ex when a potentialvdifference is applied to the terminals o-f the piezoelectric material.-v If such a square unit were supported at its center of gravity and flexed by applying a potential difference to its terminals, the exure would be such that points lying on the diagonals of the square would remain stationary while all other points of the unit would move out of the plane defined by the diagonals, points lying on one pair of opposite quadrants moving in one direction out of the plane while points lying in the other pair of opposite quadrants move in the opposite direction. It will thus be appreciated that the unit assumes a saddle shape double curvature. In an elongated rectangular unit, it will be understood that the unit will iiex along its length in a manner which appears to be mere simple bending since the bending along its length predominates while the bending across its width is negligible. Piezoelectric unit 38 in Figs. 6 and 7 iiexes in this manner, and conversely is iiexed by the levers in'a manner simulating mere simple bending.

When the above differences between the torque unit and the bender unit are recognized, it lloecomes obvious vvthat the units may be used interchangeably in a device oi the kinds described above by merely coupling the connecting links of the levers to the appropriate points.

, It should be recognized that to be operative for the purpose of interconverting electrical and mechanical vibratory energy, it is necessary 'that the piezoelectric unit be properly supported; that is, the support must permit the points of the unit at which the connecting links are coupled to move relative to the support. In addition, the points of support must be disposed relative to the moving points so as to maintain a condition of substantial equilibrium between all forces acting on ythe unit. This condition may be described by saying that the' unit is reactively supported, since the forces acting on the unit through the supports react against the forces which tend to displace the unit, thus causing the unit to be flexed. For example, in Figs. 1 through 5, the reactive supports consist of one pair of levers which oppose the action of the other pair of levers. The reactive nature of the support thus provided is particularly apparent since it will be noted that thefour levers cooperate to hold the center of gravity substantially stationary. The unit accordingly flexes or is exed as though it weresupported at its center of gravity in the manner described above; i. e., asthough it were flexing in free space. In Figs. 6 and 7, the unit is reactively supported substantially at its midpoint since the support opposes the motion induced by the lever system. In Figs. 8 and 9, torsional forces are induced in the unit by the 1ever system, the torsion being the result of a mechanical couple. The unit is reactively supported at the opposite end by supports which induce a mechanical couple opposing any couple established by the pair of actuating levers.

Referring again to the lever constructions of Figs. 1 through 5, I call attention to another feature inherent therein which is worthy of note. As already noted above the center of gravity remains substantially stationary, and it will be observed that the piezoelectric unit is exed in substantially the same manner as it would be flexed if unsupported and actuated by Aa voltage applied to its terminals. This being so, the torsional forces resulting from the inertia of the unit as it exes are equal in magnitude but oppositely directed, and consequently neutralize each other yin so far as the stresses on the supports are concerned. This feature assumes importance in devices which by their nature require the use of heavy piezoelectric units since the'inertia eects of such heavy units may become so great, if not balanced out in the above manner, that even an extremely rigid housing member may be set into vibration. Furthermore, these inertia effects may greatly increase the stresses on the lever assembly, its connecting link, and its fulcrums, thereby necessitating the use of dimensions therein which could be materially reduced if the inertia effects were balanced out. Since in this construction the piezoelectric unit flexes in the same manner as it would if unsupported, its frequency of mechanical resonance, neglecting mass loading of the lever mechanism. is the same as the free resonance of l the unit. The resonance frequency of the piezoelectric unit supported in this manner is usually from two to eight times higher than it` would be if the unit were mounted in a manner to permit inertia forces to react on the supporting structure. Thus for any required resonance frequency this construction permits the use of a,

larger piezoelectric unit. Large units are advantageous since they have lower electrical impedance and are usually easier to handle in assembly.

While the invention has been disclosed by reference to certain specific embodiments thereof, it is to be understood that it is not limited thereto, but that various modiilcations and changes, within the scope of the appended claims. may be made therein without departing from the spirit and essence of the invention.

What I claim is:

1. A device particularly adapted for interconverting electrical and mechanical vibratory energy, said device comprising: a piezoelectric crystal unit of the flexing type having a pair of terminals; reactive supporting means for said unit, said supporting means permitting a selected pair of spaced points on said unit to be displaced with respect thereto and to do work. when a potential difference is applied to the terminals of the unit: a pair of fulcrumed levers adapted to transfer energy between a first and a second coupling point thereon. said levers having their first coupling points connected to a common link, and having their` second coupling points connected to the pair of spaced points on the unit, the fulcrums oi' the levers being disposed relative to the coupling points of their respective levers to cause the common link to be displaced when a potential difference is applied to the terminals of the unit, and

conversely 'to displace said points relative to thel supporting means when the common link is displaced.

2. A device as claimed in claim 1 wherein the reactive supporting means comprises at least one additional fulcrumed lever having its nrst coupling point connected to the common link, and having its second coupling point connected to a point on the piezoelectric unit distinct from the said pair of spaced points thereon.

3. A device as claimed in claim 1 wherein the reactive supporting means consists of a second pair of fulcrumed levers having their nrst coupling points connected to the common link, and having their second coupling points connected to the piezoelectric unit at points which are distinct tially at the other two corners.

5. A devioo as claimed in olim 1 having vibration-responsive means coupled tothe common.

link.

6. A device as claimed in claim l wherein the reactive supporting means consists of a second pair of levers having their first coupling points connected to the common link of the first pair of levers. and having their second coupling points connected to the piezoelectric unit at points which are distinct from the said pair of spaced points and which permit said levers reactively to support said unit. the device including a common support for the fulcrums of the two pair of levers and for a duid-pressure-responsive diaphragm connected to the common link,

7. A device as claimed in claim 1 wherein the said pair of levers are connected to the ilexing type units at points which are substantially equidistant from the center oi.' gravity on a line passing therethrough and which lie on opposite sides of both the major crystalline axes which pass through the center of gravity: and wherein the reactive supporting means consists of a support disposed substantially at the center of gravity.

8. A device as claimed in claim 1 wherein said piezoelectric unit is a four-cornered unit of the torque type and has two opposite' corners connected to the second coupling points of the said pair of levers. and is reactively supported substantially at its other two corners.

9. A device as claimed in claim 1 wherein said piezoelectric unit consists of asubstantially square unit: and wherein said pair ot levers are of the first class and are fulcrumed on supports secured to a rigid casing member providing circumferential support for a disc diaphragm adapted to be vibrated by nuid pressure acting on a surface thereof, the second coupling points of saidpair of levers being connected to opposite cornersof the piezoelectric unit; and wherein said reactive supporting means consists-:of a second pair of levers of a dinerent class and are fulcrumed on supports secured to said casing member and have their first .coupling points connected to the said common link, and have their second coupling points connected to the other two corners oi' the pienoelectric unit; the common link of said two pairs of levers being disposed approximately on a line passing through the geometrical center of the diap hragm and the geometrical center of the unit and coupled to said diaphragm substantially at its geometrical center, saidconstruction having the characteristic that nuctuations of the iiuid pressure acting on the diaphragm ilex the pieaoelectric unit and generate a potential diiference at its terminals. while vibrations acting elsewhere on the deviceare substantially ineective in exing the piezoelectric unit.

10. A device as claimed in claim 1 wherein the reactive supporting means consists of a second `pair of fulcrumed levers having their nrst coupling points connected to the common link. and having their second ,coupling points connected to the piezoelectric unit at points which are distinct from said pair of spaced points and which permit said levers reactively to support said unit, the fulcrums of said second pair of levers being disposed relative to the coupling points thereof to permit the center of gravity of the unit ,to remain substantially stationary when the unit is flexed by the application of a potential difference to its terminals or by displacement of the common link.

CHARLES K.. GRA'VIEY.

CERTIFICATE OF CORRECTION.

Patent No. 2,207,559. July 9,.19l;0.

CHARLES K. GRAVLEY.

It ia hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 141;, for "tongue" read --torque unit; and that the said Letters Patent should be read with this correction therein that the same ma'y conform to the record of' the case in the Patent Office.

Signed and sealed this 17th dey of September, A. D. 1914.0.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION.

autant No. 2,207,559. July 9,.19ho.

CHARLES K. GRAVLEY.

It ia hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line )4.14, for "tanque" read -torque unit; and that the said Letters Patent should be read with this correction therein that the same ma'y conform to the record of' the case in the Patent Office.

signed and sealed this 17th day of september, A. D. 19140.

Henry Ven Arsdale, (Seal) Acting Commissioner of Patents. 

